Click Status to see various statistics about the currently-running server.

Click Connections button to see and adjust the connections between applications and audio hardware.

JACK operates with special real-time privileges. You must add all JACK users to the jackuser group. Learn how to add users to groups in Chapter 22, Users and Groups of the Fedora Deployment Guide, available at http://docs.fedoraproject.org. Do not add a user to the jackuser group if they will not use JACK.

By default, QjackCtl configures JACK to use the 'default' sound card. The 'default' sound card uses all features of the ALSA driver even though JACK provides the same features. To save memory and processor time, you should configure QjackCtl with a specific sound card.

Open a terminal. In GNOME, choose Applications → System → Terminal. In KDE, click on the application launcher, then choose System → Konsole.

Execute this command: cat /proc/asound/cards

The cat program outputs a list of sound cards in your computer that looks similarly to the output below.

In this example output, the square brackets surround the name of the sound card. The names of the sound cards in this example output are SB and MobilePre.Identify the name of the sound card that you want to use. If you do not see your sound card in the list outputted by cat, then your computer does not detect it.

Start QjackCtl.

Click Setup to open the "Setup" window.

In the 'Interface' text box, type the name of your preferred sound card with "hw:" in front. With the sound cards listed above, you might write hw:MobilePre.

Save your settings by exiting QjackCtl. If you want to use JACK, restart QjackCtl.

Using the Jack D-Bus Interface

Control applications such as qjackctl can be configured to interact with the jack
sound server using D-Bus. This has the advantage of allowing jack to request the
release of the sound device from Pulseaudio (see below).

jackdbus is shipped in a separate package. To install this (as root) with DNF do:

Integrate JACK with PulseAudio

JACK uses D-Bus to gain control of the sound card from PulseAudio. PulseAudio should grant control, giving JACK control of it. However this does not work at the time of writing.

There are three approaches that can be taken:

1. Disable PulseAudio altogether.

Create the file ~/.pulse/client.conf conataining the line:

autospawn = no

This will prevent PulseAudio starting again. Run:

pulseaudio -k

To stop the current instance.

2. Disable PulseAudio only when starting JACK (this is not practical for JACK-DBus):

If you use QJackCtl to start JACK then just add this to the 'Execute script on startup' entry in the Setup|Options menu:

pulseaudio -k

This will kill PulseAudio for long enough for JACK to take control of the sound card, though PulseAudio will restart unless the 'autospawn=no' configuration mentioned above has been set.
If starting JACK directly at the command line you can use pasuspender:

pasuspender jackd [options...]

With PulseAudio disabled you will not have sound for system sounds, browser
media playback, etc (which is usually what you want). But it is possible:

Now when you start JACK, PulseAudio is killed long enough for JACK to take control of the sound card. When PulseAudio restarts it's able to communicate via JACK. If you want you can use the connection menus to disconnect or reconnect it as needed.

Running Jack in Realtime mode

NOTE: This is obsolete and kept for informational purposes. The changes suggested below are done automatically by Jack installation (/etc/security/limits.d/95-jack.conf) and no manual modification of the pam configuration should be necessary.

The JACK server jackd has the capability to run in a real-time mode
which greatly decreases the chance of audio glitches. The real-time mode
is enabled by passing the -R or --realtime option to jackd when starting
the server. It is only possible to run jackd in real-time mode as a
non-root user by modifying your PAM configuration, PAM stands for
Pluggable Authentication Modules and is the primary authentification
mechanism used on Fedora. The primary source of PAM documentation can be
found at the following at http://www.kernel.org/pub/linux/libs/pam/

The specific PAM configuration file that needs to be modified is
/etc/security/limits.conf and it controls the system resource limits. It
is important to understand that modifying the resource limits
configuration files can decrease the security of your system.
Documentation specific to the resource limits PAM module can be found at
http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam-6.html#ss6.12

Alternatively, one can create a new file

/etc/security/limits.d/<priority>-<name>.conf

that contains the resource limits for individual users and/or groups. The
jack-audio-connection-kit package that comes with Fedora contains the file

/etc/security/limits.d/99-jack.conf

that does this job for you.

The resource limits that need to be changed to allow jackd to run in
realtime mode are named rtprio and memlock. To increase the limits for a
specific user named fred you would add the following to
/etc/security/limits.conf

fred - rtprio 70
fred - memlock 50000

The value of rtprio can be set in the range 0 - 99 where any value
greater that 0 will allow the user to change the scheduling policy to
"real-time". By default the JACK server requires a minimum rtprio
setting of 20 but jackd will accept a command line parameter -P or
--realtime-priority which will change the minimum required value of
rtprio that is needed, but the default of 20 is nearly always
sufficient.

The appropriate value for memlock is dependent on the amount of memory
present in the system but a minimum value of 50000(50MB) and a maximum
value of half the available memory can be used as a rough guideline.

To verify that the resource limits have been modified you can use the
bash built-in ulimit command, for example:

rtirq prioritizes the following IRQ's by default using a simple pattern matching method. This will result in elevating the realtime priority of any interrupt containing the string "snd" (in the above example it will match snd_hda_intel)

If you use other devices you prioritize them by inserting a matching string in RTIRQ_NAME_LIST after rtc.
For example, RTIRQ_NAME_LIST="rtc usb2 snd i8042" could be used to match a USB class complaint device on USB bus 2, or RTIRQ_NAME_LIST="rtc firewire_ohci snd i8042" to match a firewire_ohci or firewire_core device interrupt

To start rtirq via systemd:

sudo systemctl start rtirq.service

To start rtirq by default when you login to your desktop:

sudo systemctl enable rtirq.service

realTimeConfigQuickScan

realTimeConfigQuickScan is a utility which inspects the realtime capabilities of your system. If any errors are found it suggests methods to rectify the problem.

realTimeConfigQuickScan has both text and grpahical user interfaces. Either select QuickScan from your desktop menu or run realTimeConfigQuickScan from the terminal.

An example using an untuned system. This machine has an onboard intel sound device on IRQ 45

$ SOUND_CARD_IRQ=45 realTimeConfigQuickScan
== GUI-enabled checks ==
Checking if you are root... no - good
Checking filesystem 'noatime' parameter... found - warning
/ does not have the 'noatime' parameter set
/mnt/rawhide does not have the 'noatime' parameter set
/home does not have the 'noatime' parameter set
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#filesystems
Checking CPU Governors... CPU 0: 'ondemand' CPU 1: 'ondemand' CPU 2: 'ondemand' CPU 3: 'ondemand' CPU 4: 'ondemand' CPU 5: 'ondemand' CPU 6: 'ondemand' CPU 7: 'ondemand' - not good
Set CPU Governors to 'performance' with 'cpufreq-set -c <cpunr> -g performance'
See also: http://linuxmusicians.com/viewtopic.php?f=27&t=844
Checking swappiness... 60 - not good
** vm.swappiness is larger than 10
set it with '/sbin/sysctl -w vm.swappiness=10'
See also: http://linuxmusicians.com/viewtopic.php?f=27&t=452&start=30#p8916
Checking for resource-intensive background processes... none found - good
Checking checking sysctl inotify max_user_watches... < 524288 - not good
increase max_user_watches by adding 'fs.inotify.max_user_watches = 524288' to /etc/sysctl.conf and rebooting
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#sysctl.conf
Checking access to the high precision event timer... not readable - not good
/dev/hpet found, but not readable.
make /dev/hpet readable by the 'audio' group
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#hpet
Checking access to the real-time clock... not readable - not good
/dev/rtc found, but not readable.
make /dev/rtc readable by the 'audio' group
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#real-time_clock
Checking whether you're in the 'audio' group... yes - good
Checking for multiple 'audio' groups... no - good
chrt: failed to set pid 0's policy: Operation not permitted
Checking the ability to prioritize processes with chrt... no - not good
Could not assign a 80 rtprio value. Set up limits.conf.
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#limits.conf
Checking kernel support for high resolution timers... found - good
Kernel with Real-Time Preemption... not found - not good
Kernel without real-time capabilities found
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#installing_a_real-time_kernel
Checking if kernel system timer is set to 1000 hz... found - good
Checking kernel support for tickless timer... found - good
== Other checks ==
Checking filesystem types... ok.
not found.
** Warning: no tmpfs partition mounted on /tmp
For more information, see:
- http://wiki.linuxmusicians.com/doku.php?id=system_configuration#tmpfs
- http://lowlatency.linuxaudio.org

An example of a fully tuned system. This system uses threadirqs rather than PREEMPT_RT kernel and has a USB class compliant device on USB bus 2 (thus the shared IRQ warning)

== GUI-enabled checks ==
Checking if you are root... no - good
Checking filesystem 'noatime' parameter... - good
Checking CPU Governors... CPU 0: 'performance' CPU 1: 'performance' CPU 2: 'performance' CPU 3: 'performance' - good
Checking swappiness... 10 - good
Checking for resource-intensive background processes... none found - good
Checking checking sysctl inotify max_user_watches... >= 524288 - good
Checking access to the high precision event timer... readable - good
Checking access to the real-time clock... readable - good
Checking whether you're in the 'audio' group... yes - good
Checking for multiple 'audio' groups... no - good
yes - good.
Checking the ability to prioritize processes with chrt... yes - good
Checking kernel support for high resolution timers... found - good
Kernel with Real-Time Preemption... not found - not good
Kernel without real-time capabilities found
For more information, see http://wiki.linuxmusicians.com/doku.php?id=system_configuration#installing_a_real-time_kernel
Checking if kernel system timer is set to 1000 hz... found - good
Checking kernel support for tickless timer... found - good
== Other checks ==
Checking filesystem types... ok.
ok.
** multiple devices found at the sound cards' IRQ

CPU Performance Governing

Fedora uses cpupower rather than cpufreq-set as quoted in the realTimeConfigQuickScan output.
As of Fedora 17, cpupower is part of the kernel-tools package.

To set your CPU Governors to performance, edit /etc/sysconfig/cpupower to read: